How much flash space should I reserve for device tree blobs?

How much flash space should I reserve for device tree blobs?

[Brian Hutchinson]

Reflecting back on the famous quote of "Nobody needs more than 640K",
how much space should I reserve in flash to 'future proof' my MTD map
in regards to dtb's?

Looking at the current flavors of dtb's built in a recent mainline
kernel, the biggest one I see is 66K.

How big do you all see these getting?  I know, it's probably a loaded
question.  Would just like to bounce this off the list as a sanity
check ... measure twice cut once kind of thing.

Regards,

Brian

How much flash space should I reserve for device tree blobs?

[Jason Cooper]

Hey Brian,

+Arnd, Olof.

On Mon, Dec 15, 2014 at 03:33:50PM -0500, Brian Hutchinson wrote:
> Reflecting back on the famous quote of "Nobody needs more than 640K",
> how much space should I reserve in flash to 'future proof' my MTD map
> in regards to dtb's?
> 
> Looking at the current flavors of dtb's built in a recent mainline
> kernel, the biggest one I see is 66K.
> 
> How big do you all see these getting?  I know, it's probably a loaded
> question.  Would just like to bounce this off the list as a sanity
> check ... measure twice cut once kind of thing.

There's probably better folks to answer this than me (former powerpc
folks, eg Arnd, Olof), but since I've had to consider it as well, I'll
tell you my thoughts.

The biggest unknown is the addition of binary blobs.  If those are a
consideration for your board(s), then you need to reserve quite a bit
more space.

For example, a binding update could start including the firmware for an
SD wifi card.  However, in the interest of retaining backwards
compatibility, Linux would still be able to load from /lib/firmware.
So, if you couldn't update your dtb, you'd still have a working wifi
card.

If you aren't concerned about binary blobs, then I'd say you're fine
with 128k or so.

Most of the scenarios I had were with 2MB NOR flash.  barebox is usually
in the range of 200k, and there isn't enough room left for the kernel,
so I had plenty for the environment and the dtb.

hth,

Jason.

How much flash space should I reserve for device tree blobs?

[Arnd Bergmann]

On Tuesday 16 December 2014 07:22:24 Jason Cooper wrote:
> 
> On Mon, Dec 15, 2014 at 03:33:50PM -0500, Brian Hutchinson wrote:
> > Reflecting back on the famous quote of "Nobody needs more than 640K",
> > how much space should I reserve in flash to 'future proof' my MTD map
> > in regards to dtb's?
> > 
> > Looking at the current flavors of dtb's built in a recent mainline
> > kernel, the biggest one I see is 66K.
> > 
> > How big do you all see these getting?  I know, it's probably a loaded
> > question.  Would just like to bounce this off the list as a sanity
> > check ... measure twice cut once kind of thing.
> 
> There's probably better folks to answer this than me (former powerpc
> folks, eg Arnd, Olof), but since I've had to consider it as well, I'll
> tell you my thoughts.

I thought that these days a lot of boot loaders can just access file
systems. If you use a FAT or ext4 file system (on block based storage)
or something like UBI (on raw flash) to store both the kernel and DT,
you should be able to avoid hardcoding sizes.

It also depends on how much space you have total. On an eMMC, I would
just round up to whole megabytes, but if you have only 4MB of NOR flash
available, you have to be more conservative.

	Arnd

How much flash space should I reserve for device tree blobs?

[Robert Schwebel]

On Mon, Dec 15, 2014 at 03:33:50PM -0500, Brian Hutchinson wrote:
> Reflecting back on the famous quote of "Nobody needs more than 640K",
> how much space should I reserve in flash to 'future proof' my MTD map
> in regards to dtb's?
> 
> Looking at the current flavors of dtb's built in a recent mainline
> kernel, the biggest one I see is 66K.
> 
> How big do you all see these getting?  I know, it's probably a loaded
> question.  Would just like to bounce this off the list as a sanity
> check ... measure twice cut once kind of thing.

With barebox, we put the devicetree into the root filesystem and search
it via an freedesktop.org bootspec entry these days.

rsc 
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